This specification relates to semiconductor devices including semiconductor photodetectors.
Semiconductor p-n junctions can be used to construct photodiodes for detecting photons. An avalanche diode is one example of such photodiodes. Single-photon avalanche diodes (SPADs) are designed to detect single photons and can be used in a variety of applications, including biological, military and biometric applications. In some other image sensors, such as CMOS Active Pixel Sensors or CCDs, photon flux received by the sensing area is translated into collected charge and the collected charge is then read out as a detector output signal. Geiger-mode SPADs are different. In Geiger-mode SPADs, the output is either a digital pulse corresponding to a photon-arrival event or analog information corresponding to the precise time-of-arrival of the photon. As such, SPADs can be used both as low-light-level imagers, as well as in more exotic applications such as Time-Correlated Single-Photon Counting and Fluorescence Correlation Spectroscopy.
Geiger-mode SPADs were pioneered by Sergio Cova in the 1980's based on research from the early days of semiconductor research at Bell Labs and later at Shockley Laboratories on the avalanche behavior of semiconductor junctions. In the Geiger mode, the pn junction is biased above its breakdown voltage and, in the absence of free charge carriers, the pn junction in the Geiger mode is not conductive. When a free carrier enters the high-field region of the pn junction in the Geiger mode, the carrier is accelerated by the electric field and the accelerated carrier collides with the lattice to cause impact ionization. The impact ionization produces an electron-hole pair. The electron and the hole are accelerated in opposite directions to collide with the lattice and release additional carriers in a chain reaction. Above the breakdown voltage, carriers are generated in the junction faster than they are extracted to cause an avalanche breakdown process. The avalanche breakdown must be quickly quenched in order to prevent heating and irreversible damage to the pn junction.
The charge carriers produced by the avalanche can be electrically sensed with high timing accuracy, and the avalanche is quickly quenched to prevent damage to the p-n junction. The pn junction is then reactivated by recharging the junction in excess of its breakdown voltage. Small SPAD pixels provide such benefits as reduced dark current, lower jitter, shorter dead time and improved spatial resolution. Yet, the area of some small CMOS SPAD pixel demonstrated to date is about 3,250 μm2, approximately 1000 times larger than commercially-available CMOS Active Pixel Sensor (APS) pixels. This relatively large pixel area for a CMOS SPAD pixel is in part due to the unique structure of the SPAD pixel, which is structured to withstand high voltages and high current densities and to be electrically isolated from the sensing circuitry. The large pixel size can make it difficult or unfeasible to manufacture large arrays of such detectors, as required in commercial imagers. In other applications which do not require arrays of SPADs, such as in Fluorescence Correlation Spectroscopy, the percentage of pixel area which is sensitive to incoming photons (fill factor) is also constrained by the aforementioned unique structure. In addition, in applications requiring large pixels (for easing the optical alignment), such as in microscopy systems, the quality of the manufacturing process can be critical for limiting false counts.